As a VTR for recording and reproducing image signals of a standard television system, there is known an apparatus which has a circuit configuration shown in FIG. 7 including a magnetic reproducing head amplifier 53 (hereinafter called first prior art).
In the first prior art, image signals reproduced from a magnetic tape (not shown) by a magnetic head 51 located on a rotary drum are entered to a magnetic reproducing head amplifier 53 through a rotary transformer 52 composed of a rotor on the rotary drum and a stator on a fixed drum, and are released therefrom through a reproducing amplifier 54 and an equalizer 55 after being amplified by the magnetic reproducing head amplifier 53.
An equivalent circuit obtained by reducing the circuit shown in FIG. 7 at the secondary side(stator side) of the rotary transformer 52 is shown in FIG. 8, where a stray capacitance at the rotor side is omitted. In the figure, C.sub.in represents an input capacitance of the magnetic reproducing amplifier 53; in the same manner, C.sub.s1 as well as C.sub.s2 : stray capacitance to ground of signal wiring; L.sub.H : a head inductance of the magnetic head 51; R.sub.H : a head resistor of the magnetic head 51 (corresponding to a head loss); n: turn ratio; the secondary side to the primary side (stator side to rotor side) of the rotary transformer 52; L.sub.2 : a coil inductance at the secondary side of the rotary transformer 52; L.sub.T : a leakage inductance of the rotary transformer 52.
In the above, since L.sub.T &lt;&lt;n.sup.2 .multidot.L.sub.H &lt;&lt;L.sub.2 are satisfied within the range of n=1 to 1.5 (n: turn ratio), the equivalent circuit shown in FIG. 8 is simplified into the one shown in FIG. 9. Now C.sub.s represents the reduced input capacitance of an amplifier including the input capacitance C.sub.in, stray capacitances to ground C.sub.s1 and C.sub.s2, and other capacitances. The resonance peak frequency f.sub.o of the above resonance circuit is normally set at the vicinity of the upper limit of FM carrier transmission band as shown in a curve (A) in FIG. 10, for example, by the use of an externally attached capacitor 56. However, these days it is rear to especially install the capacitor 56, and in the case where the capacitor 56 is not used, the sensitivity is heightened by increasing the coil inductance L.sub.H.
Generally, in the magnetic reproducing head amplifier 53 of a standard television system, when there is a resonance peak frequency f.sub.o within a transmission band for FM carrier, it is difficult to simultaneously perform an inverted compensation for resonance characteristic (to restore a preferable characteristic of amplifier frequency) and a head aperture compensation as its original purpose, that is, an equalization compensation, only by the use of the equalizer 55. Therefore, at first, in order to dump a resonance characteristic, dividing resistors 57 and 58 as well as a negative feedback resistor 59 are installed as shown in FIG. 7, and by properly setting the values of these resistors, without decreasing the CN ratio for the FM carrier and with dispersions of the reproducing sensitivity or other characteristics of the magnetic head 51 being controlled, a gain frequency characteristic of the magnetic reproducing head amplifier 53 is flattened as shown in a curve (B) in FIG. 10, and then the equalization compensation in high frequency range is performed by FM equalization in the equalizer 55.
Moreover, in order to shift the upper limit of the frequency transmission band for an FM carrier to the higher frequence side, by adding a capacitor 60 for widening band width, as shown with dotted lines in FIG. 7, the gain-frequency characteristic (B) of the magnetic reproducing head amplifier 53 is shifted to have a wider band as shown in a curve (D) in FIG. 10, and such a resonance characteristic as shown in a curve (C) is obtained by the equalization compensation at the equalizer 55.
On the other hand, in a VTR for recording and reproducing wide-band image signals such as HDTV baseband signals having a luminance signal band width of 20 MHz, for example, TCI (Time Compressed Integration) signals having a baseband width 12 MHz are divided into two channels in its band width, and frequency modulation is performed on their respective carriers. However, FM carrier spectrum for each channel has a wide-band distribution characteristic ranging from a lower vicinity of 6 MHz to a higher vicinity of 30 MHz even in the first side band, supposing, for example, 18 MHz is a central carrier frequency, and therefore it is necessary for the magnetic reproducing head amplifier to have a wide-band frequency characteristic to deal with the above band width.
One example of the configuration of a VTR for HDTV is shown in FIG. 11. Referring to the example of the configuration (hereinafter called the second prior art) in FIG. 11, in reproduction, an FM carrier for HDTV reproduced by a combined-use recording/reproducing head 62 is entered to a head amplifier 64 through a rotary transformer 63. Between the rotary transformer 63 and the head amplifier 64, is installed a recording-reproduction changeover switch 64, which connects the circuit to PB sides in reproduction as shown in the figure and, on the other hand, changes the connection to REC sides in recording, and thereby an FM recording current supplied from a recording amplifier (not shown) is released to the combined-use recording/reproducing head 62 through the rotary transformer 63. In the VTR for HDTV, as the rotary transformer 63, that of a flat-plate type is normally used, which has a turn ratio of 2:2 (rotor side to stator side).
Furthermore, referring to another example of the configuration of a VTR for HDTV (hereinafter called the third prior art) shown in FIG. 12, a rotary drum has a combined-use recording/reproducing head 66, a first recording-reproduction changeover switch 67, a head amplifier 68 and a second recording-reproduction changeover switch 70 built therein, and the second recording-reproduction changeover switch 70 is connected to a reproducing amplifier 73 through a rotary transformer 71 and a third recording-reproduction changeover switch 72. Also in this case, the recording-reproduction changeover switches 67, 70, 72 are connected to PB sides in reproduction and, on the other hand, change the connections to REC sides in recording.
In another example of the configuration of a VTR for HDTV (hereinafter called the fourth prior art) shown in FIG. 13, a magnetic head for recording 74 and a magnetic head for reproduction 75 are separately secured to a rotary drum, and a head amplifier 76 is also installed in the rotary drum. The magnetic head for recording 74 is connected to a recording amplifier 78 through a rotary transformer 77, and on the other hand, the head amplifier 76 is connected to a reproducing amplifier 81 through a rotary transformer 80.
Hereupon, as described above, with respect to FM carrier for carrying HDTV signals, whose transmission frequency band has its upper frequency limit of 30 MHz, in order to maintain a stable output amplitude characteristic in an amplifier at a step downstream, it is favorable not to contain the resonance peak frequency f.sub.o of the head amplifier input circuit within the FM carrier transmission band in the output-frequency characteristic of a magnetic reproducing head amplifier.
In the meantime, since noise generated in the head amplifier is substantially constant, the CN ratio for the FM carrier may be improved when an input carrier level to the head amplifier is raised by increasing the turn ratio n for the rotary transformer. However, when the turn ratio n is increased, there arises a problem in that the resonance peak frequency f.sub.o is reduced. For example, referring to the second prior art (FIG. 11), in the case where the inductance of the combined-use recording/reproducing head 62 is 1 .mu.H and the turn ratio n for the rotary transformer 63 is 3:2 (stator side:rotor side), the input reduced capacitance C.sub.s of the head amplifier 64 including the input capacitance C.sub.in, the stray capacitances to ground C.sub.s1, C.sub.s2 and other capacitances amounts to 13 pF, and only that much amount brings the resonance peak frequency f.sub.o to less than 30 MHz.
Generally, in the case where the head amplifier input capacitance C.sub.in is great, or its stray capacitances to ground C.sub.s1, C.sub.s2 are great, as shown in a curve (E) in FIG. 14, the resonance peak frequency of the input circuit gets reduced from f.sub.o lying out of the FM carrier transmission band (a curve (F)) to f.sub.o ', and therefore is normally included within the FM carrier transmission band (a curve (B)). In that case, in addition to a required equalization compensation (see a curve (H) in FIG. 15) by an equalizer (see FIG. 7) at a step downstream according to the aperture characteristic of the recording/reproducing head 62 (see a curve (G) in FIG. 15), an inverted compensation for the resonance characteristic of the resonance peak frequency f.sub.o ' must be simultaneously performed by the same equalizer, and consequently an equalization for a steep rise in high frequency range as shown in a curve (I) in FIG. 15 is inevitable, thereby causing it very difficult to maintain a stable characteristic of the equalizer.
Furthermore, referring to the third prior art(FIG. 12), by installing the head amplifier 68 in the rotary drum, since the stray capacitances to ground can be excluded from the head amplifier input reduced capacitance C.sub.s, the C.sub.s gets reduced by C.sub.s =C.sub.in, and therefore the resonance frequency is maintained high. However, in this case it is necessary to dispose the first and the second recording-reproduction changeover switches 67, 70 both at the front and the rear of the head amplifier 68 inside the rotary drum, and thereby a complicated structure is required.
Moreover, the fourth prior art(FIG. 13) has a configuration wherein the first and the second recording-reproduction changeover switches 67, 70 are omitted from the third prior art. Also in this case, the resonance peak frequency can be maintained high; however it requires a complicated structure like the third prior art in that the magnetic head for recording 74 and the magnetic head for reproduction 75 are necessary as respective separate devices in addition to the doubled number of channels for the rotary transformers 77, 80.